Method of vertically fracturing formations in wells



"nited rates METHOD OF VERTICALLY FRACTURING FORMATIONS IN WELLS Application January 26, 1955, Serial No, 484,311

7 Claims. (Cl. 166-22) N Drawing.

This invention pertains to the, fracturing of subsurface formations traversed by wells and more particularly it per tains to a method by which the fractures thus produced extend longitudinally of the well bore. This application is a continuation-in-part of my co-pending application Serial Number 383,119,.fild September 29 1953, now abandoned.

In the art'of fracturing a reservoir structure, a low penetrating fluid is placed in the well bore opposite the formation to be fractured. The fluid penetrates to a certain extent the formation adjacent to the well bore. Upon the application of hydrostatic pressure {upon the fluid in the well bore, the fluid which has penetrated the pores of the formation is subjected to such pressure and exerts an ex- P mlins rzree 9n. thefcr na c us t o break down or fracture.- Such fractures'are usually in a horizontal P P ticula y inlayered u sur ce stra a sinc th formation accepts'fluidpenet-ra-tion to the greatest degree between such layers andis weakest in that plane. A disadvantage of horizontal fracturing, particularly in formations of subsurface stratajof widely' varying vertical permeability and low horizontal permeability, is that only one stratum is opened'to the well bore with little or no improvement in the availability of flow from other strata. If vertical fractures can be created in a formation, the fractures will extend across all strata making available increased flow from all strata. to the well bore,

It is a principalobject of my invention to provide a method of fracturing the subsurface strata traversed by a well bore so that the major portion of the fractures are vertical.

Other objects of the invention will appear as the description proceeds.

7 Experimental studies which are in agreement with theory have demonstrated that the hydrostatic pressure of a liquid confined in a well bore under conditions such that the liquid will not readily penetrate the formation exerts a tensile stress in a direction tangential to the periphery which is greater than that exerted in an axial direction of the well bore.

Broadly stated this invention comprises the method of efiecting a vertically extending fracture in a layered subsurface formation penetrated by a well bore which comprises: V

. (a) Filling the well bore in the area of said layered formation with a fluid mass which when maintained under hydrostatic pressure less than that required to effect any fracture of the formation will substantially seal off the well bore surface of the formation againstrfluid ingress,

(b) maintaining such fluid mass under a pressure less than that which will effect any fracture of said formation until the well boresurface of the formation is substantially sealed off against fluid ingress; then 7 a (0) increasing the hydrostatic pressure on said fluid mass until said formation is fractured.

From the foregoing it will be Qbserved that an essential feature ofthis invention is to effect fracturing or, stated in another way, to'snbject the formation traversed by the 2 well to a pressure sufliciently great to cause fracturing thereofunder such conditions that the applied hydrostatic pressure in the well bore is not transmitted to the fluid in the formation in the vicinity of the well bore. There are many types of fluids which could be used for the purpose of effecting a seal of the well bore surfaces of the formation to prevent fluid ingress or the transmission of the frac V turing pressure into the formation. This seal or sheath on the well bore wall may be the result either of the presence of materials which act purely'in a physical manner or by the presence in the fluid of materials which act in a chemical or quasichemical manner; a

As illustrative of materials which function in a purely physical manner, I may mention materials such as ground nut shells, shredded leather waste, ground mica, and fibrous materials such as cotton and asbestos linters.

Materials which act in a chemical or quasichemicajl fashion are well known and are generally referred to as low fluid loss materials or penetration retardants such as, for

example, blown asphalt or other high molecular weight or cumstances the use alone of materials of either of the' groups referred to above may be satisfactory, generally it will be found best to employ an admixture of the two.

Various other materials are available which will even-' tually build up an impenetrable sheath on-the well wall where fracturing is'to take place but the types of materials referred to above are preferred because they are'effective to very quickly build up such a sheath as a result of a minimum penetration of the carrier liquid into the formation to the extent that the amount of carrier liquid required to penetrate the formation to build up the sealing sheath is reduced, the over-all cost of effecting the fracture is co rrespondingly reduced both as regards the time required by the operation as well as the materials employed.

Since generally fracturing is desired in the well area adjacent the bottom of thehole, myimproved method can be carried out conveniently by placing a charge of low fluid loss material in the bottom of the well and then fill.-

ing the well bore area with a conventional oily liquid such.

a displacement into the formation of enough of the lowfluid loss material in the bottom of the well bore to provide the necessary sheath. By thus using a material which is able to form a sheath for the quantity necessary to fill the well bore, the mechanism of generating and applying the necessary hydrostatic fracturing pressure is simplified and he verll cost of he opera i n reduced to a When the procedure just outlined is followed, that is by placing a charge of the sheath forming material in the bottom of the hole in advance of filling the well with an oily liquid, it will usually be found best to employ a sheath forming material which will not gel or otherwise set fora reasonable length of time so that the operators may have ample time within which to fill the well bore with the oily liquid and make the necessary pump connections and the like by the pumpers subsequent to generating the necessary hydrostatic pressure. For these, reasons it will generally be Pr f r bl to us a sheath f rmin m erial whi h is non-gelling since when such material is used it will not .be necessary after the fracturing operation has been completed to introduce a gel breaking substance into the well.

In ear -yin .0u the. rnu cves process of this inyention e h a h f rming ma erial is introduced into the bottom of h ho e and a pecker s t in the well bore to seal on the area which is to be subjected to the fracturing pressure. The pressure generating oil is then introduced and the pressure built up to initiate the formation of'the filter cake or seal on the well bore face of the formation to be fractured. The initial pumping should be continued until a surface pressure of a few hundred pounds is reached at which the pumps are stopped and the rate of pressure decline is then observed. As the sheath forming material bleeds into the formation depositing the sheath or seal on the face thereof the pressure will of course decline so that after a short waiting period after the application of the initial pressure, the pressure drops and then the pressure should be restored to the original value by pumping into the well an additional quantity of pressuregenerating oil. There will be observed that as the sheath or cake builds up the rate of pressure decline substantially decreases.

These steps of repeatedly restoring the initial pressure should be repeated often enough until there has been a substantial decline in the rate of pressure loss. The pressure should then be increased a few hundred pounds and the step-wise process of re-establishing the pressure is then repeated.

After the surface pressure has been raised to about 1,000 p. s. i. g. in a series of steps as outlined above and when the pressure decline rate has decreased to the point where the pressure drop in about one hours time is only about 50 p. s. i. g. it is safe to assume that a sufiiciently impermeable sheath has been built up on the wall of the well bore since pumping equipment available for the purpose of establishing the hydrostatic fracturing pressure is such that a sufficient quantity of oil may be pumped into the well at such a rate that the pressure will rise rapidly enough to the point of fracture without there being any substantial transmission of such fracturing pressure through the sheath or cake and into the fluid which has penetrated the structure adjacent the well.

Instead of proceeding step-wise in building up the sheath or cake in the manner described an alternative procedure is to hold the pressure constant at each pressure step by continuous pumping and to observe the decline in the pump rate necessary to maintain the pressure. This alternative procedure will be found particularly convenient at the lower pressures, that is, during the early stages of formation of the sheath on the wall of the well bore and during which time there may be substantial penetration of fluid into the formation surrounding the Well bore. Generally, however, after the sheath has been built up to a substantial extent the step-wise procedure outlined above will be found most convenient for use.

As an illustration of actual operating conditions which have been employed in efiecting a fracture in accordance with the method of this invention the pumps were started and the pressure built up to 200 p. s. i. g. By continuous pumping the pressure was maintained at 200 p. s. i. g. for 15 minutes after which the pressure was increased to 400 p s. i. g. By continuous pumping the pressure was maintained at 400 p. s. i. g. for another 15 minutes after which it was increased to 600 p. s. i. g. During successive 15-minute intervals the pressure was raised first to 800 p s. i. g. where it was held for 30 minutes. After holding the pressure constant at 1,000 p. s. i. g. for 30 minutes and noting that there was no substantial pressure drop during that interval, the pumps were engaged whereupon the pressure built up rapidly until it reached 2,800 p s. i. g. at which point it fell off rapidly indicating that fracture had occurred.

The following are specific examples of sheath forming compositions which are satisfactory for use in carrying out the process of this invention.

'4 Example 3 Diesel fuel oil bL- 1 Postdodecylbenzene sulfonate lbs 8 Postdodecylbenzene sulfonate as used in these examples was the sodium salt of the sulfonic acid produced by sulfonating the top fraction of a postdodecylbenzene bottoms overhead distillate. Physical properties of tins postdodecylbenzene bottoms overhead distillate fraction before sulfonating are as follows Specific gravity at 48 C" Average molecular weight 300 A. S. TEBMPD-158 Engler F. B. P F 738 Refractive index 1.4902 Viscosity at F. S. S. U Bromine number .40

2 Th dodecylbenzene sulfonate used was the sodium salt of the sulfonic acid produced by sulfonatin a blend of monoalkylbenzenes, sometimes called dodecyl enzene, obtainable from Continental Oil Company under the trade name of Neolene 400. Physical properties of Neolene 400 are as follows:

Specific gravity at 16 C 0.8742 Average molecular weight 23 A. S. T. M. D148 Engler I B. P F 535 5 F" 545 10 F 550 50 F 560 90 F 580 97 F 592 F. B. P F 603 Refractive index at 20 C 1.4885 Viscosity at 20 C 14 In general the material which is placed in the well for the purpose of sealing the wall of the well bore to prevent fluid penetration should have a fluid loss of less than 5 cc. in 30 minutes as determined by A. P. I. R. P. No. 29 test conducted, however, in the temperature range from 90 F. to 350 F., that is, well bore conditions rather than at room temperature. Such a low fluid loss material may be secured by the use of the chemical addition agents mentioned in Examples 3, 4, and 5 above without the use of any solid filter cake material such as the ground nut shells which are a component of Examples 1 to 3 above. It will be noted that instead of using ground nut shells, any of the other fibrous materials previously mentioned may be used. As indicated, however, entirely saitsfactory results can be secured, at least in certain formations, by the use of penetration retardant materials which are entirely chemical in nature.

The chemical penetration retardant material identified above as postdodecylbenzene sulfonate may be found to be sensitive to the presence of substantial amounts of water. When water will be encountered, then the use of materials such as dodecylbenzene sulfonate will make it possible to tolerate certain amounts of water which would not be permissible otherwise. The postdodecylbenzene sulfonate material is substantially oil soluble and of limited Water solubility whereas the dodecylbenzene sulfonate is of limited oil solubility and substantially water soluble. The combination of these two materials is, as previously indicated, particularly effective when Water is encountered in the well.

Instead of using any of the oils mentioned above as the carrier for the penetration retardant material, entirely satisfactory results can be secured by the use of either straight crude oil such as is available in the field or conventional oil base drilling muds of which many are commercially available.

After the structure has been fractured as evidenced by a very rapid drop in the pressure in the well, it will generally be desirable to then pump into the structure an oil containing a substantial proportion of sand or another propping material so as to fill the opening caused by the fracture with a structure which will permit the oil to drain into the well bore.

As an example of the improved results secured by the method of this invention as compared with conventional fracturing techniques as heretofore employed, three comparable wells were selected in a Wyoming field having a 60-foot sandstone pay section at a depth of about 5,000 feet. These wells on production tests prior to treatment were producing 66 barrels of oil per day. Wells 1 and 2 of this group were fractured using conventional technique, that is, by merely pumping into them a 40 A. P. I. gravtiy crude oil having a viscosity of 1 centipoise. The structure fractured at a surface pressure of 1,000 p. s. i. g. This treatment did not increase the production rate of either of these wells. Well No. 3 was then fractured by the method of the present invention by first putting into it an oil base composition consisting of diesel oil containing pounds of blown asphalt and 25 pounds of ground walnut shells per gallon of oil. The well was then pressurized using diesel oil by first slowly building up the pressure to 200 p. s. i. g. and then at -minute intervals increasing the pressure by increments of 200 p. s. i. g. until a pressure of 1,000 p. s. i. g. was reached which did not substantially fall off over a period of 30 minutes. The pressure was then rapidly increased and fracturing occurred at 2,800 p. s. i. g.

After this third well was fractured, 4,200 gallons of the oil-sand mixture used in fracturing wells 1 and 2 were pumped into well No. 3 to extend and prop the fracture produced. This treatment of well No. 3 increased its production rate to a stabilized rate of 120 barrels per day. The occurrence of a particular fracture in well No. 3 was clearly indicated by the fact that fracturing occurred at very considerably higher pressure than in wells 1 and 2 and by the increased production from well No. 3.

Laboratory experiments also show that the method of the present invention is superior to any of those disclosed in the prior art for producing vertical fractures. In these experiments inch diameter holes were drilled down the center of 4-inch diameter cores after which the cores were fractured using different fracturing methods. The results were as follows:

Fracturing Method Type of Fracture Diesel oil plus 10 pounds blow-n asphalt 90% vertical, 10% horizontal. and 25 pounds ground walnut shells It is apparent that many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof. The

6 specific embodiment described is given by way of example only and the invention is limited only by the terms of the appended claims.

What is claimed is:

1. The method of effecting a vertically extending fracture in a layered subsurface formation penetrated by a well bore which comprises forming an impermeable sheath on the surface of the well bore by pumping a fluid mass into the well bore in the area of said formation, said fluid mass possessing a fluid loss of less than 5 cc. in 30 minutes as determined by an API RP No. 29 Test conducted at a temperature within the range of from to 350 F., alternately pumping and stopping the flow of a second fluid mass into the well bore, thus gradually increasing the hydrostatic pressure upon said first mentioned fluid mass until the pressure drop over an hours time after the flow of the said second mentioned fluid ceases does not exceed 5 percent of the hydrostatic pressure reached during the pumping operation and then rapidly increasing the hydrostatic pressure on said first mentioned fluid mass until said formation is fractured.

2. The method of effecting a vertically extending fracture in a layered subsurface formation penetrated by a well bore which comprises forming an impermeable sheath on the surface of the well bore by pumping a fluid mass into the well bore in the area of said formation, said fluid mass possessing a fluid loss of less than 5 cc. in 30 minutes as determined by an API RP No. 29 Test conducted under well bore conditions, alternately pumping and stopping the flow of a second fluid mass into the well bore, thus gradually increasing the hydrostatic pressure upon said first mentioned fluid mass until the pressure drop over an hours time after the flow of the said second mentioned fluid ceases does not exceed 10 percent of the hydrostatic pressure reached during the pumping operation and then rapidly increasing the hydrostatic pressure on said first mentioned fluid mass until said formation is fractured.

3. The method of claim 1 wherein said first mentioned fluid mass comprises diesel fuel oil, oxidized asphalt, and ground nut shells.

4. The method of claim 1 wherein said first mentioned fluid mass comprises mineral oil, blown asphalt, and ground nut shells.

5. The method of claim 1 wherein said first mentioned fluid mass comprises diesel fuel oil, postdodecylbenzene sulfonate, and ground nut shells.

6. The method of claim 1 wherein said first mentioned fluid mass comprises diesel fuel oil and postdodecylbenzene sulfonate.

7. The method of claim 1 wherein the said first mentioned fluid mass comprises diesel fuel oil, postdodecylbenzene sulfonate, and dodecylbenzene sulfonate.

References Cited in the file of this patent UNITED STATES PATENTS 2,547,778 Reistle Apr. 3, 1951 2,645,291 Voorhees July 14, 1953 2,650,195 Cardwell et al. Aug. 25, 1953 2,699,213 Cardwell et al. Jan. 11, 1955 2,734,861 Scott et al. Feb. 14, 1956 

1. THE METHOD OF EFFECTING A VERTICALLY EXTENDING FRACTURE IN A LAYER SUBSURFACE FORMATION PENETRATED BY A WELL BORE WHICH COMPRISES FORMING AN IMPERMEABLE SHEATH ON THE SURFACE OF THE WELL BORE BY PUMPING A FLUID MASS INTO THE WELL BORE IN THE AREA OF SAID FORMATION, SAID FLUID MASS POSSESSING A FLUID LOSS OF LESS THAN 5 CC. IN 30 MINUTES AS DETERMINED BY AN API RP NO. 29 TEST CONDUCTED AT A TEMPERATURE WITHIN THE RANGE OF FROM 90 TO 350*C., ALTERNATELY PUMPING AND STOPPING THE FLOW OF A SECOND FLUID MASS INTO THE WELL BORE, THUS GRADUALLY INCREASING THE HYDROSTATIC PRESSURE UPON SAID FIRST MENTIONED FLUID MASS UNTIL THE PRESSURE DROP OVER AN HOUR''S TIME AFTER THE FLOW OF THE SAID SECOND MENTIONED FLUID CEASES DOES NOT EXCEED 5 PERCENT OF THE HYDROSTATIC PRESSURE REACHED DURING THE PUMPING OPERATION AND THEN RAPIDLY INCREASING THE HYDROSTATIC PRESSURE ON SAID FIRST MENTIONED FLUID MASS UNTIL SAID FORMATION IS FRACTURED. 